Scratch

It's tricky, but it's possible. Here's how:
First you need to make your player's shoes a certain colour, but you must make sure that nothing else in the game is the same colour as their shoes.
Next, make the moving platform. The player will be able to stay on the moving platform as long as you press the arrow keys as it's moving along.
Finally, once you've set the colour of your player's shoes and made the moving platform, you can add this script to your player:

[blocks]<when green flag clicked>[/blocks]
[blocks]<forever if>[/blocks][blocks]<< [blocks]<color[ shoe colour ]is over[ sky[/blocks] <and> [blocks]<( [blocks]<x position>[/blocks] <=> -100 )>[/blocks] >>[/blocks]
[blocks]<broadcast[ fall down ]and wait c>[/blocks]

This script means that if the player's shoes are touching the sky and it's x position is low down, it will fall down and the level will start again. Remember to add a "when I receive" block somewhere, with a script of what you want the game/player to do when it receives the "fall" message.
I hope this helps! If you need me to explain broadcasting to you, or you don't understand, just let me know and I will help you!
Happy Scratch-ing! 

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http://scratch.mit.edu/projects/Dan_Test/396312